A162941 Number of reduced words of length n in Coxeter group on 7 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.

1, 7, 42, 252, 1491, 8820, 52185, 308700, 1826160, 10802925, 63906150, 378045675, 2236381350, 13229622000, 78261652875, 462967596000, 2738748634125, 16201445085000, 95841881782500, 566965863568125, 3353964722666250

Offset

0,2

Comments

The initial terms coincide with those of A003949, although the two sequences are eventually different.

Computed with MAGMA using commands similar to those used to compute A154638.

Links

Table of n, a(n) for n=0..20.

Index entries for linear recurrences with constant coefficients, signature (5, 5, 5, -15).

Formula

G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(15*t^4 - 5*t^3 - 5*t^2 - 5*t + 1)

Mathematica

coxG[{4, 15, -5}] (* The coxG program is at A169452 *) (* or *) LinearRecurrence[ {5, 5, 5, -15}, {1, 7, 42, 252, 1491}, 30] (* Harvey P. Dale, Mar 13 2018 *)

Crossrefs

Sequence in context: A102594 A053142 A214941 * A094168 A163345 A163923

Adjacent sequences: A162938 A162939 A162940 * A162942 A162943 A162944

Keyword

nonn

Author

John Cannon and N. J. A. Sloane, Dec 03 2009

Status

approved